US2855895A - Apparatus for coating paper - Google Patents

Description

1 BUQNS m; 2,855,895

v v APPARATUS FOR COATING PAPER File'd Dec. 22', 1954 PRIORART Thomas 0. Burns William H.Murvin atet 2,855,895 Patented Oct. 14, 1958 APPARATUS FOR COATING PAPER Thomas Burns and William H. Marvin, Hamilton, Ohio, assignors to The Champion Paper and Fibre Company, Hamilton, Ohio, a corporation of Ohio Application December 22, 1954, Serial No. 477,015

3 Claims. (Cl. 118-407) This invention relates to paper coating methods and apparatus. It has particular reference to processes in which fluid coating composition is smoothed into a uniform film on the surface of a paper web by means of a doctor at a location where the paper web is carried on the surface of a rotating roll, and in which a pool of the fluid coating composition is maintained on the surface of the moving web immediately in advance of the doctor. Such coating processes have been successfully used in the coating of large tonnages of paper as apart of its manufacture on the paper making machine. In the commercial use of this process, however, when speeds have been increased above the order of 800 feet per minute, increasing trouble with streaks in the finished coating has been encountered. It is the object of this invention to reduce or eliminate this difficulty and thus make possible higher operating speeds with coaters of this type.

In the said coating process the movement of the paper web in the bottom of the pool of coating tends to carry the coating with it. Since only a minute quantity of the coating is able to pass under the doctor on the surface of the paper, most of the moving coating is held back by the doctor and tends to flow along the surface of the doctor in a direction away from the surface of the roll. It then runs backward over the top of the forwardly moving coating, forming a continuous circulation of the coating in the surface of the pool. This circulation becomes more rapid and the pool of coating becomes visibly more and more violently agitated as the speeds are increased, and this is believed in some way to be responsible for the increasing occurrence of streaks in the finished paper as the speed of operation is increased. We have now found that we can greatly improve the operation and substantially reduce or eliminate the occurrence of streaks in the finished paper, and thereby make higher operating speeds commercially possible, by diverting this outward flow of the coating from its normal path along the surface of the doctor and directing it, at least in part, back towards the center of the pool before it reaches the surface. This, we find, can be conveniently accomplished by providing one or more baffles along the face of the doctor in the path of the stream of coating which tends to flow outwardly along said surface, all as hereinafter more particularly described.

The invention will now be further described with reference to the accompanying drawing in which:

Fig. l is a fragmentary diagrammatic view, partially in section, of a coating device of the type described before application of the present invention thereto, indicating roughly how it is believed that the coating in the pool is caused to circulate due to the movement of the paper which constitutes the bottom of the pool.

Fig. 2 is a fragmentary diagrammatic illustration of the pertinent portions of the web-carrier roll and the doctor blade which enclose the pool of coating material,

and showing the modifications which characterize the present invention.

Fig. 3 is a view similar to Fig. 2, but illustrating a modified form of baffle for deflecting the stream of coating flowing outward along the doctor.

Fig. 4 is a similar view illustrating the use of a plurality of battles for deflecting the outwardly flowing current.

Referring first to Fig. l, the paper web 11 is carried on the surface of the roll 12 which is rotating in the direction shown by the arrow. The doctor blade 15 is mounted on a suitable framework 16 which may, if desired, be provided with a pivotal mounting, as indicated at 17, on which it can be swung away from roll 12, for cleaning, threading, or other purpose. Fluid coating composition is maintained in a pool 21 formed in the angular bight between the surface of the paper web 11 carried on roll 12, and the doctor 15. The pool 21 of coating is maintained at the desired depth by continuous or intermittent feeding as through a shower pipe 22.

The continuous movement of paper web 11, on roll 12 along the bottom of the pool 21 naturally tends to drag along with it the adjacent portions of the coating, which tendency apparently becomes more and more pronounced as the speed is increased. Since only an extremely thin layer of coating is able to pass under doctor 15 on the surface of the paper web 11, the major part of the coating which is following the movement of the paper web 11, is wiped off of the paper by doctor 15 and flows away from the roll 12 along the surface of doctor 15, as indicated crudely by the arrows in Fig. 1. As the speed is increased, the increase in this current in the pool 21 of coating becomes apparent from an increasingly prominent boiling up of the coating as illustrated at 23. As the speed is stepped up, streaks in the coating on the finished paper become increasingly evident until they reach a point where further increases in speed can not be made without rendering a prohibi: tively large portion of the production commercially unacceptable.

While the cause of these streaks is unknown, it appears to be related in some way to the turbulence of the coating in the pool 21. It has been conjectured that at least some of the streaks may be due to air bubbles, assumed to have been entrained in the coating at the turbulent surface of pool 21, which may be carried onto the paper and drawn under the doctor, resulting in elongated areas of relatively little or no coating on the paper surface. It has also been conjectured that the turbulence may cause local variations in hydrostatic pressure, immediately in front of the working edge of the doctor, which may also be at least in part responsible for some of the streaks observed in the finished paper as speeds are increased. It is also thought that other and less clearly understood causes which may be more or less closely related to turbulence in pool 21, may be at least partly responsible for the observed defects.

Referring next to Fig. 2, which illustrates one embodiment of the present invention, paper web 11, carrier roll 12, doctor 15 and holder 16 are as illustrated in Fig. l. A new element, baffle 31, which has been added in Fig. 2, is shown as clamped in holder 16, along with the doctor blade 15. The presumed effect of the baffle 31 on the circulation of the fluid coating composition in pool 21, is indicated by the arrows in Fig. 2 which show how it is believed that the upwardly flowing coating stream is deflected by baffie 31 and directed back towards the relatively quiescent coating material in the center of the pool,

'before it reaches the upper surface where it appears as a boiling up of substantially less intensity than in the absence of baffle 31. With this arrangement, regardless of theories as to cause, the number of streaks appearing 3' on the finished paper at any given speed is greatly reduced or entirely eliminated, thus making possible operation at substantially higher. speeds than are otherwise possiblebefore the-numberand severity of defects renders. the operation uneconomic.

In. Fig. Sisillustratedabafile. 36 of a different type which has. been found. useful with coating. compositions of lower viscosity, thansometimes. used with the device illustrated in Fig. 2. -Here1the portion.37 of the bafiie is closer to thezpap'er. surface than is the case in Fig. 2. A substantially smaller proportion of the coating which is carried. along. with the paper web can enter under bafilesurface 37. Thisgit is.- believed, is then reversed in directionby theqaction of, doctor blade,15 and baffie face-.37. and meets the remaining liquid being carried alongwith the :paper, head-on, thus;=dissipating the harmful energy well below the -surface :of ithe pool, the assumed subsurface currents being again indicated by the arrows.

In other cases, a pluralityiof. ba'files camsometimes be effectively combined,- asillustratech in Fig. 4, when because of speed, type of coating;composition being used, or other cause, it'is' found ditficult to'secure a suflicient reduction in turbulence by use of a single baflde.

Because of the' wide variety of coating compositions and paper types used in the industry,.as well as variations in the size, angular arrangements, andproportions of the coater'elements which'in themselves form no part of the present invention, it is not possible to give any one set of' dimensions and locations for the baffles which are necessary to secure the best results in any one instance. However, the man skilled in the art, by following the instructions-herein given, will haveno difficulty in securing a substantial reduction, if not complete elimination, of the streaks'occurring in the finished paper when operating a coater of this type a't any-giVen-speed, or in substantially increasing the -speedat which the machine can be operated while keeping streaks in the finished paper below practical commercial limits.

When operating thecoating device as illustrated in Fig. 1, it will be observed-as speed is increased, that'the, turbulence in pool 21 increases'as shown by an increase in size and'height-of the boiling up 23. As speeds are increased, particularly with coatings of lowerviscosity, the whole surface ofpool 21 becomes violently agitated'and coating material is splashed out of the pool, splattering adjacentparts of the machine and even spotting thepaper. At the same time, streaks in the finished paper become increasingly evident until they render the operation commercially impractical.

To correct this condition, a bafile may be inserted as illustrated in Fig. 2' or Fig. 3. If the improvement is insufficient, a broader bathe, i. e. one which projects farther out from the surface of the doctor, may be substituted or an additional baflie may be added as illustrated in Fig. 4. It has also been observed that faulty operation also takes place when the surface of pool 21 becomes too quiescent. If this is found to be the case the second bafiie, if.present, may be removed. Otherwise a narrower baffle,,i. e. one which does not project so far from the surface of the doctor, may be substituted, or the angle of the baffle may be alteredso that the outer edge thereof is farther from roll 12, until the appearance of streaks in the paper is reduced to a minimum or completely eliminated, at the given speed. When thisv is the case the surfaceof the pool 21 has, in general, been :found to be neither quiescent nor violently turbulent, but-onlymoderately agitated; When a different coating composition, or a rougher or smoother paper stock is substituted for that in use, 'even though the machine speed remains the same,- ,-itis generally true thata change in the-form, size; -1ocation,,or numberof baifies may be required to give bestf results;

To assist in the, understanding of the invention, the following example is given: The, roll: 12 was. approximately inches in diameter. The doctor 15 contacted the roll 12 at a point about degrees above the horizontal and formed an angle of about degrees with the tangent to the roll surface at that point. The pool 21 of coating was then about 5 inches deep at the deepest point. A conventional type of aqueous coating composition was being applied at about 47 percent solids to a conventional coating stock. At speeds up to something over: 700 feet-per, minute, no significant difficulty was encountered due to occurrence of streaks in the finished paper. As speeds-were, increased to and above 800 feet per minute; however, such. streaks became an increasingly serious defect in the resulting product. A baflieisimilarto that'illustrated at.31 in Fig. 2 was then inserted. The bafile projected approximately inch from, and formed'a'n angle' of about 90 degrees with, the surface of the doctor 15 at a point about 1% inches from the surface of the roll 12. It then became possible to operateattspeedsjup, to 900 feet per minute without any significant-formation of streaks.

When, usingacoating of lower viscosity on a coating device-of essentially the same dimensions, best results were secured by use, of a baffle such, as illustrated in Fig. 3 in which thee /si inch width face 37 was in contact with doctor 15 at a distanceof about 1 inch from the surface of roll'12. In cases. where the turbulence in the pool 21 was. still. too great and. streaks were still found in the paper, a second battle was used as, illustrated in Fig. 4, whichstill-further-widened the range of coatings which-could-be:satisfactorily. used at this speed.

In: any case, the man skilled in the art, bearing these instructions; in: mind, can by. watching the agitation of the surface ofpool 21, andcorrelating this with the degreeof perfection or=imperfection of the finished product, readily determine-whether'more or less bafiling action is required to give the desired result with the coating composition and-paper raw stock and onthe specific coating machine-at=handand=at the speedrequired, within commercial limits.

It is thus-apparent=that-thepresent invention, when applied to coating materialsand apparatus of the type set forth, serves to better adaptthem to use forthe coating of paper in'the courseof its manufacture on modern high speedpaper makingmachines by making, possible higher speeds for the coating operation without sacrifice of quality andYorby improving-the quality of the product without the necessityfor-any reduction inspeed'.

We claim:

1. In a paper coatingdevicewhich comprises a continuously'rotatablepaper-webcarrying roll, and a doctor mounted in position for one edge thereof to contact a paper web on the surface of said roll to form an angular bight adapted to maintain a pool ofliquid coating composition in contact with a paper web. on the surface of said roll immediately in advance of its passage under said doctor; the improvement which comprises the pro- VlSlOIl of means submerged in said pool for diverting a flow of liquid from along the surface of said doctor towards the surface of said. pool and directing it into subsurface portions of said; pool.

2. In apaper coating device which comprises a'rotatablc paper-web-carryingroll,- a doctor mounted adjacent the surface of said roll for maintaining'a pool of liquid coating composition in contact with paper carried on the surface of said roll'and immediately ahead of said doctor, and

' for allowingonly athin layer of coating composition to remain on the surface of thepaper whenit is carried by said roll undersaid doctor, the improvement which comprises the combination with the doctor of at least one bathe extending outwardly. from and along the face of the doctor directly-into the. main body of liquid in said pool, well below the surface thereof, where it is adapted to-prevent coating-' composition in said pool from flowing along the face of said doctor from thepaper on said roll surface, to thesurface of saidpool and to direct it into. subsurface portions of said pool.

3. A paper coating device which comprises: a continuously movable paper-web-carrying surface, a doctor mounted adjacent thereto with one edge in position to contact a paper Web on said surface to form an angular bight adapted to maintain a pool of liquid coating composition in contact with a paper web on said carrying surface, at least one baflle means located well below the surface of the coating composition in said pool, projecting from the face of said doctor, and adapted to prevent liquid from flowing along the surface of said doctor to the surface of said pool and to direct the flowing liquid into the subsurface portion of the main body of liquid in the pool.

References Cited in the file of this patent UNITED STATES PATENTS Savage Mar. 6, 1894 Knowlton June 11, 1901 Doyton July 11, 1922 Van Derholf Sept. 6, 1927 Strauch Feb. 20, 1940 Barrett Aug. 31, 1943 Francis Apr. 20, 1948 Roehm June 28, 1949 Taylor Dec. 19, 1950 Beguin June 15, 1954 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Noo 2,855,895

October 14;, 1958 Thomas 00 .Bur'ns at al,

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters .Patent should read as corrected below.

Column 1, line 42, after ."in the", strike cut "surface of the", and insert the same before "pool" in line 43, Same columlr Signed and sealed this? 22nd day of September 1959:

(SEAL) Attest:

KARL H.) AXLINE ROBERT C. WATSON Attesting Oflicer Commissioner of Patents-

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